One of the most important problems in electronic surveillance involves the recognition of a signal in a multisignal environment. In the past, it has been common to use so-called compressive receivers for rapidly sweeping a given band of frequencies to determine the presence or existence of a signal and its frequency. However, the mere identification of the presence of a signal and its frequency is almost always insufficient to obtain the identity of the signal and thus its source. It is, therefore, important to identify some other signal parameters as a key to the source of the signal. In this invention, the parameters selected are frequency spectrum and modulation.
Historically, compressive receivers have been used as power spectrum analyzers in which an envelope detector is connected to the output of the dispersive delay line utilized with the compressive receiver. The mere detection of the envelope results only in information that a signal of a particular center frequency is present and it is difficult to obtain any further information about the particular signals. It should be noted that envelope detection also destroys most spectral information.
It is a characteristic of compressive receivers that the output of the dispersive delay line utilized with the compressive receiver produces a compressed pulse carrying what is called "residual FM" (frequency modulation). This means that the oscillations in the envelope actually change frequency towards the ends of the envelope and it was thought that because of the "residual FM", both complete demodulation and spectral analysis would be impossible. Thus, historically, the output of the compressive receiver was fed to an envelope detector so that at least the center frequency of the signal could be obtained.
It has been found that in spite of the "residual FM", the output of a compressive receiver can, in fact, be successfully demodulated and/or a spectral analysis successfully performed. In an involved mathematical computation, it was learned that with a sweep-to-sweep phase coherent variable frequency oscillator, effect of the "residual FM" is cancelled, thereby leaving a signal which is, in fact, Fourier transformable and which, in fact, can be completely demodulated without foldover. This has been proven in subsequent testing.
With this finding and a sampling rate above the valid sampling rate for signals of interest, signals previously switched to conventional receivers for demodulation and/or spectral analysis can be demodulated or Fourier transformed in the compressive receiver itself by dispensing with the envelope detector and adding demodulating or Fourier transform circuits. Thus, the modulation on a signal and its spectral signature or "fingerprint" can be obtained in real time and without further apparatus. This permits extremely rapid identification of the source of a signal wih one receiver.
Thus, it is a finding of this invention that by replacing the standard envelope detector with an FFT (Fast Fourier Transform) or other equivalent devices and that by utilizing a sweep-to-sweep phase coherent variable frequency oscillator, a Fourier transformable signal is, in fact, present at the output of the dispersive delay line, be it a conventional dispersive delay line or the wide bandwidth embodiment described hereinafter. Moreover, it is a finding of this invention that complete demodulation is, in fact, possible using standard demodulators in place of the envelope detector, along with the sweep-to-sweep phase coherent variable frequency oscillator.
In compressive receiver systems with appropriate band pass or inphase and quadrature (I and Q) processing, the theoretical required sampling rate is one-half the commonly accepted Nyquist rate for base band sampling. Thus, the minimum valid sampling rate is equal to the bandwidth of the signal of interest. As a practical matter in compressive receivers, a factor of 1.4.times.the bandwidth assures unaliased sampling.
When the present day compressive receiver is utilized in a sweep mode in which the incoming signal is heterodyned with a fast sweeping local oscillator signal, rapid sweeping of the compressive receiver ordinarily results in a sampling rate too slow, e.g. below the "valid sampling" rate, for complete demodulation and/or spectral analysis for certain signals of interest. This slow rate is primarily due to the limited bandwidth of present day dispersive delay lines used with the compressive receiver.
By way of background, in general, the compressive receiver is one which employs a linear variable frequency oscillator. This oscillator is swept such that its signal, when mixed with an incoming signal (should one be present), produces a linear FM signal. The linear FM signal is coupled to a dispersive delay line which time compresses the linear FM signal. When the output of the dispersive delay line is displayed as a function of time, the position of the compressed pulse on the time axis correlates to the frequency of the incoming signal. This type receiver can provide for sampling of the frequencies within, for instance, a 1.6 MHz band between 5 MHz to 6.6 MHz. For this case, the "revisit" time, that is, the time between samples of a given frequency, is typically on the order of 3.6 milliseconds.
However, with a 1.6 MHz band and with, for instance, an FSK incoming signal (frequency shift 300 Hz) being sampled only once every 3.6 milliseconds, the sampling is at less than the valid sampling rate for this FSK signal (e.g. sampling once every 3 milliseconds is required to validly sample this FSK signal). This means that certain phase shift keyed (PSK) and frequency shift keyed (FSK) signals cannot be completely demodulated. The valid sampling rate determines the critical point below which it is impossible to completely demodulate the incoming signals or do spectral analysis. In short, below the valid sampling rate there are insufficient data samples of an incoming signal and thus, insufficient information about the incoming signal for either demodulation or spectral analysis.
Once above the valid sampling rate, complete demodulation and spectral analysis of the incoming signals can be done in one receiver without the necessity of switching the incoming signals to separate conventional receivers and spectral analyzers.
In summary, it is one of the most important findings of this invention that if the variable frequency oscillator is sweep-to-sweep phase coherent, the outputs of the dispersive delay line from successive sweeps combine into a signal which is, in fact, completely demodulatable and on which a successful Fourier transform may, in fact, be performed.
With the capability of doing spectral analysis and demodulation plus the increased valid sampling rate, it is a simple matter to sweep a wide bandwidth and identify any signal which appears at the input to the compressive receiver either by demodulation or spectral analysis. This, of course, permits much more rapid identification of a source of unknown signals than has heretofore been possible.
It is, therefore, an object of this invention to provide a compressive receiver in which the spectral signature of and modulation on an incoming signal is available in real time;
It is another object of this invention to substitute for the conventional envelope detector used in a compressive receiver a Fourier transform device and to utilize a sweep-to-sweep phase coherent variable frequency oscillator, thereby to obtain the spectral signature of signals at the input of the compressive receiver;
It is a further object of this invention to substitute for the conventional envelope detector used in a compressive receiver a demodulator and to utilize a sweep-to-sweep phase coherent variable frequency oscillator, thereby to obtain complete demodulation of the signals at the input of the compressive receiver.
These and other objects of the invention will be better understood in connection with the following detailed description taken in conjunction with the appended drawings.